Micromachined cantilevers are used in atomic force microscopy (AFM) for surface scanning and for chemical, biological, and other sensing applications. Micromachined cantilevers may become the basis for specialized, ultraminiature, ultrasensitive sensors for detection of specific target chemical species such as chemical compounds, bioactive agents, or toxins. Miniaturized chemical sensors hold promise for applications needing sensitive chemical detection, quality control of materials processing, and measurements of small or limited quantities of a chemical or biological material. Resonance-based detection has been demonstrated specifically for sensing mercury vapor, ultraviolet radiation, relative humidity, magnetic susceptibility, and sub-nanogram masses.
AFM is a method of measuring surface topography on a scale typically from a few angstroms or less to a hundred micrometers or more. The technique involves imaging a sample through the use of a probe or tip suspended from one end of a microcantilever. A surface is probed with the tip, and the interaction between the tip and sample is measured. Physical topography, surface chemistry, charge density, magnetic properties, local temperature and other surface properties can be analyzed.
Lasers can be used for optical detection of cantilever movement, though AFM systems using lasers require comparably high power on the order of milliwatts, need alignment, have limited resolution, and are prone to drift because of the size of the optical path. Current systems typically require external lighting for sample illumination and setup, and are not very compact because of the long optical path, the need to have the photodetector at an ample distance from the sample, and constrained viewing and positioning systems for optical alignment. Sample testing in liquids such as water or saline solution presents additional difficulties for optical sensing due to aberrations and refraction of the light beam traversing the fluid.
Lasers have been used to detect frequency changes of a microcantilever that is oscillated by a piezoelectric transducer, as taught in “Microbar Sensor,” Wachter et al., U.S. Pat. No. 5,445,008 issued Aug. 29, 1995. Oscillation frequency changes are detected by a center-crossing photodiode that responds to a laser diode beam reflected from the microcantilever surface resulting in an output frequency from the photodiode that is synchronous with the microcantilever frequency.
AFM systems using microcantilevers and laser detection have been used for analyzing explosive gas molecules adsorbed onto the microcantilevers, as described in “Microcantilever Detector for Explosives,” Thundat, U.S. Pat. No. 5,918,263 issued Jun. 29, 1999. Analysis can be made of the laser beam reflected by the heat-induced deflection and transient resonant response of the microcantilever.
Because power efficiency is important to the size, lifetime, and utility of a cantilever sensor, alternative detection schemes to those using lasers have been proposed. Notably, capacitive systems are being developed to monitor cantilever deflection. While a low-power option, capacitive cantilever sensing is most suitable under vacuum conditions to avoid excessive air damping between the two electrodes of the capacitor. Other limitations of capacitive schemes include small tolerances for fabrication and coatings of a cantilever, a reliance on a small sensing gap size, and difficulties with sensing in liquid solutions.
In an alternative sensing scheme to optical detection or capacitive sensing, AFM systems in a vibrating or tapping mode may use actuated piezoresistive cantilevers. Piezoresistive sense methods are more compact than optical systems, though they can self-heat and cause drift. Furthermore, piezoresistive sensing typically consumes large portions of available power when used in a portable device. A typical piezoresistive cantilever, which also can use milliwatts of power, is a micro-electrical-mechanical-system (MEMS) device, micromachined from bulk silicon with a piezoelectric film patterned along a portion of the microcantilever. At the free end of the cantilever is a tip with nanometer-scale radius, optimally shaped to probe the sample surface. The microcantilever is displaced by voltage applied to the piezoelectric actuator, resulting in a controlled vertical movement of the tip. Control electronics drive the microcantilever while simultaneously positioning it vertically to track the sample topography and follow the surface features. A macro-scale position actuator may be used to null the position of the cantilever, following the topology of the sample as the probe is scanned over the surface.
Chemicals can be sensed based on frequency shifts of microcantilevers treated with a compound-selective substance, as disclosed in “Microcantilever Sensor,” Thundat et al., U.S. Pat. No. 5,719,324 issued Feb. 17, 1998. A microsensor with a cantilever attached to a piezoelectric transducer is capable of detecting changes in the resonance frequency and the bending of the vibrated cantilever in a monitored atmosphere. Upon insertion into a monitored atmosphere, molecules of a targeted chemical to be sensed attach to the treated regions of the microcantilever resulting in a change in oscillating mass as well as a change in microcantilever spring constant thereby influencing the resonant frequency of the microcantilever oscillation.
Oscillating silicon nitride cantilevered beams coated with a thin gold film have been used to detect mercury vapor in air due to changes in cantilever resonant frequency and stress levels induced in the gold overlayer as described by Thundat, et al., in “Detection of Mercury Vapor Using Resonating Microcantilevers,” Appl. Phys. Lett. 66 (13), 27 Mar. 1995, pp. 1695-1697. An uncoated microcantilever can be used for chemical sensing by exciting charge carriers into or out of surface states with discrete photon wavelengths as disclosed by Thundat, et al., in “Uncoated Microcantilevers as Chemical Sensors,” U.S. Pat. No. 6,212,939, issued Apr. 10, 2001. Attempts at DNA sequencing and detection using an AFM is described by Allen in “Method and Apparatus for DNA Sequencing Using a Local Sensitive Force Detector,” U.S. Pat. No. 6,280,939, issued Aug. 28, 2001.
An exemplary cantilever with a piezoelectric drive and a piezoresistive sense is disclosed in “Atomic Force Microscope for High Speed Imaging Including Integral Actuator and Sensor,” Minne et al., U.S. Pat. No. 5,883,705 issued Mar. 16, 1999, and “Cantilever for Scanning Probe Microscope Including Piezoelectric Element and Method of Using the Same,” Minne et al., U.S. Pat. No. 5,742,377 issued Apr. 21, 1998. When the scanning probe microscope (SPM) operates in the constant force mode, the piezoelectric element is used to control the tip-sample separation. Since the resonant frequency of the piezoelectric element is substantially higher than that of conventional piezoelectric tube scanners, much higher scan rates can be achieved. When the SPM operates in the dynamic or intermittent contact mode, a superimposed AC-DC signal is applied to the piezoelectric element, and the latter is used to vibrate the cantilever as well as to control the tip-sample spacing.
Piezoelectrically driven cantilevers have been proposed to eliminate the need for external actuators. An exemplary self-actuating cantilever is described in “Active Probe for an Atomic Force Microscope and Method of Use Thereof,” Adderton et al., U.S. Pat. No. 6,530,266 issued Mar. 11, 2003 and “Atomic Force Microscope for High Speed Imaging Including Integral Actuator and Sensor,” Adderton et al., U.S. Pat. No. 6,189,374 issued Feb. 20, 2001. This system includes a self-actuated cantilever having a Z-positioning element integrated therewith and an oscillator that oscillates the self-actuated cantilever at a frequency generally equal to a resonant frequency of the self-actuated cantilever.
In response to the growing interest in using cantilevers for chemical sensing, researchers are developing systems with multiple cantilever sensors or modular sensor array systems to characterize larger numbers of material samples more quickly. An example of an system of multiple cantilevers in a substantially linear configuration that uses individually-selectable cantilevers with a different resonance frequency for each is disclosed in “Multiprobe and Scanning Probe Microscope,” Shimizu et al., U.S. Pat. No. 6,469,293 issued Oct. 22, 2002. A modular sensor array system has been suggested for rapid deposition of sample chemicals on sensor arrays in “Sensor Array-Based System and Method for Rapid Materials Characterization,” Mansky et al., U.S. Pat. No. 6,535,824, issued Mar. 18, 2003 and “Sensor Array for Rapid Materials Characterization,” Mansky et al., U.S. Pat. No. 6,535,822, issued Mar. 18, 2003. One intended goal is to eliminate the need for multiple materials characterization machines and the need for application-specific active circuitry within the sensor arrays themselves.
In light of the discussion above, an improved system for sensing chemicals is desirable that is more compact and power-efficient than piezoresistively or optically sensed AFM cantilevers, does not require off-chip actuation for frequency measurements, does not require individual addressing of each cantilever to determine the natural frequencies, is capable of operating in liquid or gas, and generates less unwanted heat than other AFM cantilever systems. Cantilevers need to be small and light for rapid detection of minute concentrations of target chemicals and biomaterials. Therefore, a desirable method and system for chemical sensing incorporates these improvements and overcomes the deficiencies described above.